Apolipoprotein B (apoB) is a major protein constituent of lipoproteins secreted by the liver and intestine. Newly synthesized apoB may be either fully translocated across the endoplasmic reticulum (ER) membrane, resulting in a form that is assembled into lipoprotein complexes and secreted, or incompletely translocated, resulting in a transmembrane form that is rapidly proteolyzed by an ER-associated degradation pathway. Exogenous factors affect the relative levels of secreted and degraded apoB in hepatocytes. In addition, unregulated degradation may be responsible for the absence of serum apoB in patients with abetalipoproteinemia. The goal of this proposal is to analyze structural, mechanistic, and regulatory features of apoB degradation in order to better understand the ER degradative pathway and the pathogenesis of abetalipoproteinemia. The structural determinants that target apoB to be degraded will be defined by analyzing the degradation of genetically engineered polypeptides expressed in transfectants of hepatoma cell line. Serially C-terminally deleted apoB forms and chimeric proteins consisting of relevant apoB segments fused to an indicator protein will be analyzed. Once these determinants are defined, the mechanisms underlying degradation will be addressed by analyzing proteins that associate with these determinants and by measuring the effects of metabolic inhibitors on degradation. Lastly, the regulation of apoB degradation by exogenous factors and its tissue specificity will be assessed, with particular emphasis on the fate of apoB in a cell line derived from a patient with abetalipoproteinemia.